Poor sleep impairs ability to interpret facial expressions

The research, published in the journal Neurobiology of Sleep and Circadian Rhythms, found that study participants had a harder time identifying facial expressions of happiness or sadness when they were sleep deprived versus well-rested.

The sleepy participants' ability to interpret facial expressions of other emotions - anger, fear, surprise and disgust - was not impaired, however.

That is likely because we're wired to recognise those more primitive emotions in order to survive acute dangers, said lead researcher William D.S. Killgore, Professor at University of Arizona in the US.

While emotions such as fear and anger could indicate a threat, social emotions such as happiness and sadness are less necessary for us to recognise for immediate survival. 

When we are tired, it seems we're more likely to dedicate our resources to recognising those emotions that could impact our short-term safety and well-being, Killgore said.

The data used in the study was part of a larger research effort on sleep deprivation's effects on social, emotional and moral judgment. 

Killgore began the project while working as a research psychologist for the US Army.

The current study is based on data from 54 participants, who were shown photographs of the same male face expressing varying degrees of fear, happiness, sadness, anger, surprise and disgust. 

Participants were asked to indicate which of those six emotions they thought was being expressed the most by each face.

Researchers found that sleep deprived participants had a harder time, however, correctly identifying more subtle expressions of happiness and sadness, although their performance on the other emotions was unchanged.

When participants were tested again after one night of recovery sleep, their performance on happiness and sadness improved, returning to its baseline level.

While the difference in performance was not overwhelming, it's enough that it could have a significant impact in critical social interactions, Killgore said.